TypeError: Error #1009: 无法访问空对象引用的属性或方法

1、错误描述

TypeError: Error #1009: 无法访问空对象引用的属性或方法。
at FirstMap/search_clickHandler()[E:\Flash Builder\Map\src\FirstMap.mxml:17]
at FirstMap/__search_click()[E:\Flash Builder\Map\src\FirstMap.mxml:30]

2、错误原因

<?xml version="1.0" encoding="utf-8"?>
<s:Application xmlns:fx="http://ns.adobe.com/mxml/2009"
			   xmlns:s="library://ns.adobe.com/flex/spark"
			   xmlns:mx="library://ns.adobe.com/flex/mx"
			   width="100%" height="100%" xmlns:chart="chart.*">
	<s:layout>
		<s:BasicLayout/>
	</s:layout>

	<fx:Script>
		<![CDATA[
			import mx.controls.Alert;

			protected function search_clickHandler(event:MouseEvent):void
			{
				var teacher:Teacher = new Teacher();
				var workId:String = teacher.workId.label;
				Alert.show(workId);
			}
		]]>
	</fx:Script>

	<fx:Declarations>
		<!-- 将非可视元素（例如服务、值对象）放在此处 -->
	</fx:Declarations>

	<mx:HBox width="100%" height="100%" paddingBottom="10" paddingLeft="10" paddingRight="20"
			 paddingTop="10">
		<s:Panel width="50%" height="100%">
			<s:Button id="search" label="查询" click="search_clickHandler(event)" width="100%"
					  height="100%" fontSize="320"/>
		</s:Panel>
		<chart:Teacher width="50%" height="100%"/>
	</mx:HBox>

</s:Application>

3、解决办法